ECAT — Engineering College Admission Test (UET)
Take the ECAT Chemistry — Thermochemistry, Equilibrium and Kinetics (Set 7): 30 multiple-choice questions (40 minutes) with instant scoring and a full…
What this test covers
The first 5 questions of this test, with the correct answer and explanation.
Q1. Standard enthalpies of combustion (kJ mol⁻¹): C(graphite) −394, H₂(g) −286, C₂H₆(g) −1560. The standard enthalpy of formation of ethane, 2C + 3H₂ → C₂H₆, is:
Answer: B. −86 kJ mol⁻¹
Explanation: ΔH_f = Σ ΔH_c(reactants) − ΔH_c(product) = [2(−394) + 3(−286)] − (−1560) = −1646 + 1560 = −86 kJ mol⁻¹.
Q2. 50 cm³ of 1.0 M HCl is mixed with 50 cm³ of 1.0 M NaOH in a calorimeter and the temperature rises by 6.7 °C. Taking the density as 1.0 g cm⁻³ and specific heat 4.18 J g⁻¹ K⁻¹ for the solution (heat capacity of the calorimeter neglected), the enthalpy of neutralisation is:
Answer: B. −56.0 kJ mol⁻¹
Explanation: q = 100 g × 4.18 × 6.7 = 2801 J for 0.050 mol of water formed; ΔH = −2.801 kJ ÷ 0.050 = −56.0 kJ mol⁻¹.
Q3. For N₂(g) + 3H₂(g) → 2NH₃(g) at 300 K, ΔU = −90.0 kJ. Using R = 8.314 J K⁻¹ mol⁻¹, ΔH (for the reaction as written) is:
Answer: D. −95.0 kJ
Explanation: ΔH = ΔU + Δn_gRT with Δn_g = 2 − 4 = −2: −90.0 + (−2)(8.314 × 10⁻³)(300) = −95.0 kJ.
Q4. 0.100 mol of N₂O₄ is placed in a 1.00 dm³ flask and 20% dissociates according to N₂O₄ ⇌ 2NO₂ at equilibrium. The value of K_c (in mol dm⁻³) is:
Answer: A. 0.020
Explanation: [N₂O₄] = 0.080 M and [NO₂] = 2 × 0.020 = 0.040 M. K_c = (0.040)²/0.080 = 0.020.
Q5. For 2SO₂ + O₂ ⇌ 2SO₃ at 500 K, K_c = 410.5 (concentrations in mol dm⁻³). Using R = 0.0821 dm³ atm K⁻¹ mol⁻¹, K_p (in atm⁻¹) is:
Answer: D. 10.0
Explanation: K_p = K_c(RT)^Δn with Δn = 2 − 3 = −1: 410.5/(0.0821 × 500) = 410.5/41.05 = 10.0.
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Q6. For N₂ + 3H₂ ⇌ 2NH₃, K_c = 0.50 at a certain temperature. A mixture contains [N₂] = 2.0 M, [H₂] = 1.0 M and [NH₃] = 2.0 M. The reaction quotient is Q = 2.0 and the mixture will:
Q7. The solubility product of Ca(OH)₂ is taken as 4.0 × 10⁻⁶ at 25 °C. The pH of its saturated solution (K_w = 1.0 × 10⁻¹⁴) is:
Q8. The pH of 0.10 M NH₃ solution (K_b = 1.8 × 10⁻⁵; assume x ≪ 0.10; K_w = 1.0 × 10⁻¹⁴) is:
Q9. A buffer contains 0.20 M CH₃COOH and 0.10 M CH₃COONa. Given pK_a(CH₃COOH) = 4.74, the pH of the buffer (Henderson–Hasselbalch equation) is:
Q10. The pH of 0.10 M NH₄Cl solution (K_b of NH₃ = 1.8 × 10⁻⁵; K_w = 1.0 × 10⁻¹⁴; assume hydrolysis is small) is:
Q11. The solubility product of BaSO₄ is 1.0 × 10⁻¹⁰ (mol dm⁻³)². Its solubility in 0.10 M Na₂SO₄ solution (neglecting the change due to sulphate from BaSO₄) is:
Q12. Mg(OH)₂ has K_sp = 1.2 × 10⁻¹¹. At what pH does precipitation just begin in 0.012 M MgCl₂ solution (K_w = 1.0 × 10⁻¹⁴)?
Q13. For A + B → products, initial rates are: [A] = 0.10, [B] = 0.10 M, rate 2.0 × 10⁻³ M s⁻¹; [A] = 0.20, [B] = 0.10 M, rate 4.0 × 10⁻³; [A] = 0.10, [B] = 0.30 M, rate 1.8 × 10⁻² M s⁻¹. The rate constant k (in M⁻² s⁻¹) is:
Q14. A first-order reaction has a half-life of 20 min. The percentage of the reactant that remains after 50 min is:
Q15. For a second-order reaction 2A → products, t½ = 1/(k[A]₀). If k = 0.050 M⁻¹ s⁻¹ and [A]₀ = 0.40 M, the half-life is:
Q16. A reaction has Eₐ = 50 kJ mol⁻¹. Using R = 8.314 J K⁻¹ mol⁻¹, the factor by which its rate constant increases when the temperature is raised from 300 K to 310 K is about:
Q17. An exothermic reaction (ΔH = −30 kJ mol⁻¹) has a forward activation energy of 80 kJ mol⁻¹. The activation energy of the reverse reaction is:
Q18. PCl₅(g) ⇌ PCl₃(g) + Cl₂(g) is at equilibrium in a rigid closed vessel at constant temperature. Argon gas is added at constant volume. The equilibrium position:
Q19. PCl₅ ⇌ PCl₃ + Cl₂ has K_p = 2.0 atm at a given temperature. Using K_p = α²P/(1 − α²), the degree of dissociation of PCl₅ at a total equilibrium pressure of 6.0 atm is:
Q20. Which of the following processes is accompanied by an increase in entropy of the system?
Q21. A gas absorbs 800 J of heat and expands from 1.0 dm³ to 6.0 dm³ against a constant external pressure of 2.0 atm (1 dm³ atm = 101.3 J). The change in internal energy is:
Q22. In a bomb calorimeter of heat capacity 12.5 kJ K⁻¹, burning 0.0100 mol of a compound raises the temperature by 20.0 K. The molar internal energy of combustion is:
Q23. A 50 g piece of metal at 100 °C is dropped into 100 g of water at 20 °C (specific heat 4.18 J g⁻¹ K⁻¹); the final temperature is 25 °C. Neglecting heat losses, the specific heat of the metal is:
Q24. For 2N₂O₅ → 4NO₂ + O₂, the rate of disappearance of N₂O₅ is 4.0 × 10⁻⁴ M s⁻¹. The rate of formation of O₂ is:
Q25. Which plot gives a straight line for a reaction that is second order in a single reactant A?
Q26. The unit of the rate constant of a third-order reaction (rate in mol dm⁻³ s⁻¹) is:
Q27. At 60 °C, K_w for water is 9.6 × 10⁻¹⁴. The pH of pure water (neutral) at this temperature is:
Q28. 50 cm³ of 0.10 M HCl is mixed with 50 cm³ of 0.040 M NaOH. The pH of the resulting solution is:
Q29. The equilibrium constant expression K_c for 3Fe(s) + 4H₂O(g) ⇌ Fe₃O₄(s) + 4H₂(g) is:
Q30. For N₂ + 3H₂ ⇌ 2NH₃, K_c = 4.0 at a given temperature. At the same temperature, K_c for NH₃ ⇌ ½N₂ + 3/2H₂ is: